Currently, as much as about 6 billion tons a year of carbon dioxide is emitted into the air due to the use of fossil fuels in the world. It is said that since carbon dioxide exerts the greenhouse effect, an increase in the atmospheric concentration of carbon dioxide causes an increase in atmospheric temperature on a global scale so as to raise serious environmental issues, e.g., a rise in sea level and abnormal weather. On the other hand, the speed of fixation of carbon dioxide in nature is very slow. Therefore, to take measures against the carbon dioxide is an urgent necessity. Under these circumstances, Japan made a political commitment on a worldwide basis to reduce the overall amount of emission of greenhouse effect gases by at least 6% below the 1990 level in the period 2008 to 2012 based on the international agreement in the 3rd Session of the Conference of the Parties to United Nations Framework Convention on Climate Change (COP3) held in December 1997.
With respect to technologies to make effective use of carbon dioxide, synthesis of methanol serving as a fuel through a catalytic hydrogenation reaction by using a catalyst has been attempted. However, the reaction requires a temperature of 250° C. or more and hydrogen and, therefore, if a fossil fuel is used as an energy source thereof, it becomes useless since fresh carbon dioxide is generated.
It has also been considered as a workaround therefor to generate hydrogen through the electrolysis of water by using electric energy converted from natural energy and to generate methanol through synthesis of the resulting hydrogen and carbon dioxide in the presence of a catalyst. However a large-scale development of natural energy is required and, therefore, the practicability is believed to be low since the cost becomes very high.
As described above, carbon dioxide is in a low energy state and it is difficult to use as energy. However, Japan is a carbon dioxide emission power and emits as much as 1.23 billion tons of carbon dioxide a year. Therefore, to take more down-to-earth measures directed toward the reduction of carbon dioxide is an urgent necessity, and efforts must be made in combination of the development of technology for a high energy conversion efficiency, an energy-conservation policy, and the like.
Under these circumstances, attempts to separate, recover, and fix carbon dioxide without emitting into the atmosphere from thermal power plants, chemical plants, and the like which make up 60 percent of the overall amount of emission of carbon dioxide, and to reserve in the ground or under the sea have been conducted all over the world. Consequently, emergence of a high-temperature reversible reaction type material capable of repeatedly adsorbing and desorbing carbon dioxide, exhibiting a low pressure loss, and having a high adsorption-desorption efficiency is desired in order to reduce the separation and recovery costs as well.
Examples of up-and-coming clean energy include a fuel cell system by using hydrogen. Attempts to use a natural gas which is an abundant resource and in a high energy state or methane which is a primary component of a methane hydrate as a means for producing hydrogen serving as a fuel have been intensively conducted at present. Since the generation of hydrogen from methane is based on a steam reforming reaction of methane, carbon dioxide is generated as a by-product.
As described above, in a social form in which the fuel cell has become the mainstream as well, it is believed that the establishment of technologies for separation, recovery, and fixation of carbon dioxide is an inevitable issue to prevent global warming.
In order to efficiently recover carbon dioxide, it is desirable to separate a gas having as high a concentration as possible, and a method of separation before combustion has been studied, in which carbon dioxide is separated at a stage of a reformed fuel gas before combustion. Since the steam reforming reaction of methane is generally conducted within the range of 400° C. to 600° C., it is desirable to separate and recover carbon dioxide from a high-temperature gas in order to achieve separation of carbon dioxide before combustion from the viewpoint of the effective use of thermal energy as well.
Examples of previously known technologies to absorb carbon dioxide include a chemical absorption method by using β-aminoethyl alcohol or an alkaline aqueous solution, e.g., potassium carbonate, sodium carbonate, potassium hydroxide, sodium hydroxide, or lithium hydroxide; a membrane separation method by using a cellulose acetate membrane; and a physical adsorption method by using a physical adsorbent, e.g., zeolite or molecular sieve. However, these known technologies cannot efficiently separate and recover carbon dioxide from a high-temperature gas exceeding 400° C. because of limitation of heat resistance.
With respect to the technology for separating and recovering carbon dioxide from a high-temperature gas, Japanese Unexamined Patent Application Publication No. 9-99214 has introduced a technology through the use of lithiated zirconium, as a method for separating carbon dioxide by using a temperature difference as a driving source without pressure control. Japanese Unexamined Patent Application Publication No. 2001-96122 has introduced that lithium silicate having an average particle diameter of 0.1 to 10 μm is used as a material for absorbing carbon dioxide at a temperature within the range of 100° C. to 700° C. Japanese Unexamined Patent Application Publication No. 2001-232186 has introduced a method in which lithium zirconate is dispersed in lithium silicate is introduced as a method for directly separating and recovering carbon dioxide in a high-temperature exhaust gas from an apparatus to burn hydrocarbon. Furthermore, Japanese Unexamined Patent Application Publication No. 2001-170480 has introduced a technology for improving carbon dioxide absorption capability over a wider concentration range by adding an alkali carbonate to lithium silicate.
In the case where a carbon dioxide adsorption-desorption material is filled in an adsorption-desorption tower or the like and is used for adsorbing and desorbing carbon dioxide, examples of material characteristics required of the material for the structure include high carbon dioxide adsorption-desorption capability, a low pressure loss, a high thermal diffusion efficiency, and high resistance to repeated stresses, e.g., expansion and shrinkage.
However, each of the forms of carbon dioxide adsorption-desorption materials introduced by these documents is in the shape of a powder or is molded into the shape of pellets by a method of pressure molding in a mold. Consequently, in the case where these carbon dioxide adsorption-desorption materials are used actually by being filled in apparatuses, the capabilities of these carbon dioxide adsorption-desorption materials cannot be fully used since problems may occur resulting from the pressure loss, satisfactory amounts of materials may not be filled in, or heat may not be uniformly transferred throughout the carbon dioxide adsorption-desorption materials filled in.
In addition, emergence of a high-temperature reversible reaction material capable of repeatedly adsorbing and desorbing carbon dioxide is desired in order to reduce the separation and recovery cost as well. However, the material is subjected to repeated stresses of expansion and shrinkage by adsorption-desorption operations. On the other hand, with respect to the material processed by a method of pressure molding in a mold, there are very small spaces to relax the stresses. Consequently, a problem occurs in that the molded pellets cannot withstand the repeated stresses and are gradually powdered.